I am having feeding issues with a newly built AR-15 Pistol in 458 SOCOM. Rounds 3 thru 10 hang up and don't completely chamber. If I cycle them by hand, there is no issue. I replaced the action spring with a Wolff heavy one but it is still an issue. Tried 3 different mags, same. I've used only 1 type ammo, Underwood 200gr Ballistic Tip. 

I'm thinking a longer bullet tip to flatten the round out upon entry. Or maybe a lighter buffer (remove 1 weight) to speed up the bolt. Thoughts?

No direct experience but IIRC, there were some mag questions with that round...? 

All 3 mags were made for the 458 round. I just don't think the carrier is moving fast enough, although it kicks like a mule. Sounds like the mag spring is pushing the rounds up against the carrier slowing it down. Or maybe it's too fast, and the round needs to exit the mag at a controlled pace. Hmmm.

Put stock spring back in and remove some buffer weight. 

2 hours ago, PK90 said:

Sounds like the mag spring is pushing the rounds up against the carrier slowing it down

You have more mag spring pressure on the first two or three rounds and it cycles.  Gets hung up on 3rd round, with less mag spring pressure, less friction on bolt.  Bolt friction is probably not the issue.

Have you tried loading only 4 or 5 in the mag and see how it behaves?

Are there any markings on spent brass or cartridges that failed to feed?

23 minutes ago, Scorpio64 said:

You have more mag spring pressure on the first two or three rounds and it cycles.  Gets hung up on 3rd round, with less mag spring pressure, less friction on bolt.  Bolt friction is probably not the issue.

Have you tried loading only 4 or 5 in the mag and see how it behaves?

Are there any markings on spent brass or cartridges that failed to feed?

I wasn't clear. When a mag is loaded with 2 to 3 rounds, no issues. When it has more than that, the first group has the issues until the mag gets down to the last 2 rounds. I assume that the pressure against the bolt bottom is the issue.

ETA: I'd really liked to get a hold of some Buffalo Bore 500 grain round nose ammo, but it is out of stock. This Underwood ammo is pointed at 2.13" OAL. The maximum OAL for the 458 round is 2.26".

8 minutes ago, PK90 said:

I wasn't clear. When a mag is loaded with 2 to 3 rounds, no issues. When it has more than that, the first group has the issues until the mag gets down to the last 2 rounds. I assume that the pressure against the bolt bottom is the issue.

Ah, yes.  That makes much more sense.

I don’t think a heavier spring is helping that. 

You want the bolt to cycle back faster, Fuller, stronger, better. And stuff 

How does the feed ramp look?  Is it nicely machined and blended to the receiver?  There seem to be several threads on other forums for similar issues feeding past 3 rounds.  

10 minutes ago, fatty said:

How does the feed ramp look?  Is it nicely machined and blended to the receiver?  There seem to be several threads on other forums for similar issues feeding past 3 rounds.  

Fine. Yes. 

I am going to play with the buffer weights. I ordered some tungsten weights. I will go heavy then light. I'm thinking, with a heavy buffer, if the bolt recoils far enough to pickup the next round, the extra mass will send the new round home. A mass vs velocity thing. If not, maybe remove a weight in the buffer like @Zeke said, to cause a slamming effect.

No experience with 458 but some getting 260rem to work. You may be able to get there with playing with the carrier etc etc but in the end I scraped all of that and just went adjustable gas block.

25 minutes ago, Shane45 said:

No experience with 458 but some getting 260rem to work. You may be able to get there with playing with the carrier etc etc but in the end I scraped all of that and just went adjustable gas block.

That’s probably the easiest solution.

Well, there's enough gas to recoil and pickup a new round. So your saying there's too much gas and it's cycling too fast?

34 minutes ago, PK90 said:

there's enough gas to recoil and pickup a new round

Check where your brass is landing to determine if gas is over or under.

1 hour ago, Scorpio64 said:

Check where your brass is landing to determine if gas is over or under.

10 feet at 2 o'clock 

3 hours ago, PK90 said:

10 feet at 2 o'clock 

I can't say for sure because, 458 SOCOM, but on a 5.56/223 rifle, that ejection pattern means overgassed.

Hmmm. My edit isn't showing. It's more like 2:30 with no brass deflector.

1 minute ago, PK90 said:

Hmmm. My edit isn't showing. It's more like 2:30 with no brass deflector.

Brass should be falling between 3:00 and 4:30.  That's one thing.  How's your BCG, is it light weight?  Just for kicks, if you have a few spare mag springs, try nipping a coil or two and see what happens.

Your rifle has some minor issues but I'm guessing your problem is a mix of gas pressure, BCG weight, and buffer weight.  I also think there is a little magazine issue as well.  It's a problem salad.

1 hour ago, Scorpio64 said:

Brass should be falling between 3:00 and 4:30.  That's one thing.  How's your BCG, is it light weight?  Just for kicks, if you have a few spare mag springs, try nipping a coil or two and see what happens.

Your rifle has some minor issues but I'm guessing your problem is a mix of gas pressure, BCG weight, and buffer weight.  I also think there is a little magazine issue as well.  It's a problem salad.

Agree. Tossing salad

The carrier is not lightweight. I'd hate to have to change the gas block. I ordered some tungsten weights and some long nosed ammo. I hope to test it out next Friday. Thanks for the help.

3 hours ago, Scorpio64 said:

Just for kicks, if you have a few spare mag springs, try nipping a coil or two and see what happens.

Good idea.

UPDATE:

Did three total modifications prior to taking this out this morning.

1. I received some tungsten weights for the buffer. Installed all three bringing the buffer weight to 5.5 oz. I thought, if too heavy and the bolt wouldn't come back far enough to pickup the next round, I could lighten it up from there.

2. Installed Wolff XP buffer spring.

3. I noticed that the dedicated 458 metal mags (unknown brand) kept the round level (0 angle). The Lancer 458 mags pitched the round slightly upward. Orginally. neither mag worked perfectly, but I liked the Lancer body. This mag had a standard 223 follower that I did not like. The metal mag had a single feed follower. Soooo, I modified these followers to fit the Lancer bodies.

RESULT: 

The heavy weight buffer reduced the felt recoil substantially and picked up every succeeding round. The mags fed all rounds without issue. The casings fell within a four foot circle 12 feet away at 4 o'clock. No failures to feed at all. I couldn't zero it in due to 20 mph winds, but got it close enough until next time.

Thanks for all the suggestions.